1. Field of the Invention
The invention relates to an improved method for activating metal surfaces made of iron, steel, zinc, or galvanized iron or steel, aluminum, aluminized steel, and various alloys of the foregoing, respectively, prior to the phosphation of said surfaces with phosphating baths containing zinc ions, more specifically prior to low-zinc phosphation.
2. Statement of Related Art
Methods for producing phosphate layers on iron and steel surfaces by means of acidic solutions containing phosphates of polyvalent metals as well as oxidants or other accelerator components required for phosphating have long been known. Such methods are used in the manufacture of car bodies in the automotive industry to an increasing extent in order to provide improved protection from corrosion for the iron materials or steel sheets, as commonly used in the automobiles. More recently, however, electrolytically galvanized and hot-dip galvanized steels are more frequently being used in car body manufacture, while, in addition to zinc alone, zinc alloys containing iron, nickel, cobalt or aluminum as alloy coingredients are gaining increasing acceptance as surface coatings prior to phosphation.
Prior to the application of a varnish onto the aforementioned metal surfaces, which application is usually effected by electro-dipcoating today, it is common practice to clean, rinse with water and then to phosphate the work-pieces. In known phosphating methods, it is possible in a single process step to rid the metal surfaces of adhering oil, grease and other physical contaminations and at the same time to activate them for the subsequent step of zinc phosphating. The cleaning, degreasing and activating solutions are usually applied onto the metal surfaces to be treated in a spray, dip or combined spray-dip procedure. The solutions are adjusted to from a weakly acidic to alkaline pH, and contain surfactants (wetting agents, emulsifiers), builder substances (sodium hydroxide, alkali metal carbonates, alkali metal phosphates) and, optionally, silicates and borate as well as substances having layer-attenuating and activating effects, for example titanium compounds such as titanium phosphates. Such aqueous solutions, which simultaneously display cleaning and activating effects, have been described as within the scope of methods for the pre-treatment of metal surfaces prior to the phosphation thereof in U.S. Pat. No. 4,384,900 (and corresponding German Pat. No. 2,951,600) and German Pat. No. 3,213,649.
In published German patent application 12 87 892 there has also been disclosed the use of aqueous alkaline solutions containing an alkali borate, wetting agents and/or activators, which preferably are free from silicate, for treating iron and steel surfaces prior to phosphating them with a zinc phosphate solution.
Such alkali borate solutions make possible combining the steps of cleaning, degreasing and activating and are usually applied by spraying. This is why they PG,4 contain little foaming surfactant, thus avoiding excessive foam formation.
However, most recently there have been increasingly put into use low-zinc phosphating processes such as described in U.S. Pat. No. 4,265,677 (and corresponding Canadian Pat. No. 1,134,246), as well as in published German patent application No. 22 32 067. These enable distinctly improved protection from corrosion to be achieved by the usual electro-dipcoating. Such low-zinc phosphating processes respond much more sensitively to alterations in the process parameters and to contaminations introduced into the phosphating bath with the sheets to be coated. As a result, the step of activating the metal surface becomes of much greater importance. For eliminating drawbacks in the activation of the metal surfaces it has proven to be advantageous to divide the activating step from the cleaning and degreasing step. This is all the more applicable where the phosphating solution in the low zinc phosphating process is to be applied by a dipping procedure.
Stabilizing the separate activating bath constitutes a problem which to date has not yet been satisfactorily solved. Specifically, in contrast to combined cleaning/degreasing/activation, no stable activating baths can be provided which enable a sufficiently good activation of the metal surfaces to be achieved by the phosphating step over an extended period of time.
Destabilization of the activating baths is caused by entrained "hardness constituents" (electrolytes) of the rinsing water which are carried over into the activating baths from the rinsing baths conducted between the cleaning and activating steps. Such destabilization can be avoided by various routes. For example, fully deionized water can be used for the rinsing step. However, this measure would drastically increase the total cost of the process. If tap water is used in the intermediate rinsing step, the varying water hardnesses have to be taken into account. This necessitates adapting the recipe of the activating bath to the respective conditions in water supply. Thus, in response to the actual tap water conditions, conventional softeners such as phosphates, EDTA, nitrilotriacetate, citrate and/or diphosphonylated organic compounds have to be added. However, the need for continually adapting the recipe of the activating bath to the actual process parameters by itself renders this process uneconomical. Moreover, the use of tap water in the rinsing bath significantly reduces the useful life of the activating bath.
In addition in practical operation (i.e. in sequential process steps of cleaning, rinsing, activating, rinsing and subsequent phosphating), it has been observed that upon operating for an extended period of time the activation effect is relatively rapidly reduced, which gives rise to an increase in the layer weight of the phosphate layer being formed. The layer weights are low in the beginning, but after an extended period of operation increase to values which are technologically undesirable. This requires the activating bath to be permanently controlled and/or replenished.
Moreover, it was observed again that the phosphate layers formed in the subsequent phosphation had discolored stripes and spots indicating an insufficient and/or poor activation of the metal surfaces.
Furthermore, the quality of the zinc phosphate layer applied by phosphation is sufficient only within very narrow limits of the free acid contents in the phosphating bath, which limits in practical operation are sometimes hard to realize. Zinc phosphate layers could be obtained which formed a good base for the subsequent electro-dipcoating procedure only if the free acid content of the phosphating solution was maintained within narrow limits by the addition of alkali. The low acid content also results in an increased sludge production in the bath.
The drawbacks as mentioned were particularly clearly apparent in the low-zinc phosphation of zinc or galvanized surfaces such as those being recently used in the manufacture of car bodies. More specifically, the formation of white spots which were observed upon poor activation resulted in a defective coating.